Miniaturized 3D-Printed Multipass Helmholtz Photoacoustic Sensors for High-Sensitivity CO₂ Detection with Near-Infrared (NIR) Diode Lasers

Monitoring carbon dioxide (CO₂) concentrations is crucial for environmental protection and various industrial applications. This study presents a high-sensitivity photoacoustic spectroscopy (PAS) sensing system based on an innovative 3D-printed multipass (MP) Helmholtz cell designed to enhance CO₂ detection efficiency. The MP-Helmholtz photoacoustic cell, fabricated using high-precision 3D printing technology, achieves a high level of integration among a compact multipass cell, the acoustic wave detection chamber containing a microphone, and the capillary resonance components. The multipass cell, consisting of two identical spherical mirrors, generates 160 laser beams within an effective volume of 20 mL, thereby extending the effective absorption path length and accumulating excitation light power. Consequently, results indicate that the signal amplitude of this 3D-printed MP Helmholtz cell is increased by a factor of 21, compared to that of traditional Helmholtz photoacoustic cells. Utilizing a near-infrared distributed feedback laser (DFB) tuned to the CO₂ absorption line at 6361.25 cm^-1, the sensing system achieved a minimum detectable limit (MDL) of 4.5 ppmv. The corresponding NNEA was 3.0 × 10^-10 cm^-1·W·Hz^-1/2. An optimal detection limit of 330 ppbv was established with a 48-s averaging time, demonstrating robust sensitivity. Continuous monitoring CO₂ in ambient air validated the system's stability. In comparison to alternative methods for enhancing the photoacoustic effect, such as the use of Herriott cells or erbium-doped optical fiber amplifiers, the Helmholtz cell equipped with a specially designed optical multipass cell offers a highly effective and cost-efficient approach for improving resonant photoacoustic signals.